Large heat exchangers or condensers are used in a variety of applications. One of the more common uses of these devices is in electric power plants. Condensers are used in these plants to condense steam which has been generated in boilers and passed through turbines. Typically, cool water is continuously passed through an array of sealed tubes and the steam is directed to flow around and between the tubes of cool water. This results in condensing of the steam to water.
In most cases, the cooling water which is used in these condensers is drawn from an open body of water such as a lake or a river. The cooling water, as drawn from its source, contains a variety of troublesome inclusions and solutes.
Among the inclusions found in these waters are microorganisms and trash. The word "trash", as used here, refers to items of such gross size and composition that they can cause clogging if they are permitted to enter the system. This sort of clogging is known as macrofouling. Examples of trash include pieces of wood, tires, dead fish and the like. Most condenser systems use trash screens or racks at the cooling water intake point to prevent the entry of trash into the cooling water system. Unfortunately, even the best of such systems allow some trash to enter the system and this trash accumulates in the area of the inlets to the array of condenser tubes. This blocks the tubes and renders them nonfunctional. The condenser must then be shut down, drained and cleaned by hand, an upleasant and time-consuming task.
Microorganisms are, of course, present in all natural waters. Some of these microorganisms thrive in the warm environment of the condenser tubes. These microorganisms tend to adhere to the inside surface of the condenser tubes and multiply rapidly. If this process is allowed to continue, the bore of the condenser tube will eventually become occluded and its heat transfer function will be impaired. This is known as biological fouling or biofouling. Presently, it is common practice to add anti-biofouling agents, such as chlorine to the cooling water to retard the growth of these organisms. Often, these agents must be used well in excess of need in order to provide a reasonable margin of safety. This results in waste of expensive materials and the discharge of substantial quantities of these materials into the environment.
Cooling water also contains inorganic salts and other compounds which contribute to the build-up of scale on the inside of the condenser tubes. Scale formation, like biofouling can result in diminished heat transfer and the gradual occlusion of the bore of the condenser tubes. This process of gradual occlusion by biofouling and scale formation is known as microfouling.
The accurate prediction and detection of macrofouling and microfouling is a matter of some importance. If these problems can be dealt with in their incipiency, plant down-time and damage to the condenser can be avoided or minimized. Also, more accurate measurement and prediction techniques can make possible a reduction in the safety margin which must be allowed in the application of microfouling control agents.
Previously, fouling of the condenser had been inferred from an increase in the force necessary to pump cooling water through the plant condenser. This technique does not permit the detection or prediction of fouling at an early stage. It is also unable to consistently differentiate between macrofouling and microfouling. A large number of extraneous factors can influence these sorts of measurements when they are made in a plant-scale condenser. Thus, it is at best only an approximate indication of the fouling behavior of the plant condenser. Two other techniques which have been used are removing the condenser from service for visual inspection of the tubes, and monitoring turbine back pressure, circulating water temperature and the unit electrical output while the condenser is in service and comparing these three parameters to an established baseline. None of these techniques have been entirely satisfactory.
It has been known to use pilot-scale condensers to measure Overall Heat Transfer Resistance (OHTR) and to use these devices to predict microfouling. These devices are set to operate under constant conditions which are thought to be representative of plant conditions. The OHTR is calculated by well-known methods from several factors including the temperature of input and output water, the velocity of the cooling water and several others. Thus, variations in OHTR can be due to factors other than the fouling condition. These other factors can be significant in plant condensers. Because of the smaller size of pilot plants, their OHTR's are not so greatly influenced by uncontrollable variations in these factors and are thought to be useful models of plant condenser microfouling activity. However, there are limitations on the usefulness of this technique. For example, the OHTR can actually be decreased as a result of the early stages of microfouling. This can be misleading and demonstrates the limited usefulness of this technique in the early detection of microfouling. Also, OHTR measurements give little or no guidance as to whether macrofouling is developing.
The measurement of the cooling water pressure drop across the plant condenser tubes as a fouling indicator has been considered but thought to be even less useful than OHTR measurement in pilot condensers as a tool for predicting and detecting microfouling. Pressure drop measurements in plant condensers were thought to be subject to far too many variables to be measured accurately or to be useful predictions of microfouling. Guerra, et al. Use of Pilot Scale Condensers for Biofouling Measurement and Control (1980) gave some indication that the pressure drop across the pilot-scale condenser could be measured with sufficient accuracy to be useful in the prediction of microfouling. However, Guerra, et al. were never able to observe a pilot-scale condenser whose pressure-drop behavior tracked or predicted the pressure-drop behavior of a fouling plant condenser. Also, Guerra et al. were unable to distinguish between macrofouling and microfouling.